Conventionally, ink containing a colorant, such as a dye, a pigment, or the like, a humectant, a penetrant, and water has been well known as ink used for inkjet recording. However, in the case where an image is formed with the ink containing the colorant on a recording medium, such as recording paper, or the like, the water-resistivity of the image is a matter of concern. Especially when an image is recorded with ink containing a dye on plain paper, the water-resistivity of the image is very poor. (The “plain paper” herein refers to paper which is one of various types of commercially-available paper, which is especially used for an electrophotographic copying machine, and which is produced without an intention to have an optimum structure, composition, properties, or the like, for inkjet recording.)
As disclosed in Japanese Unexamined Patent Publication No. 10-212439, Japanese Unexamined Patent Publication No. 11-293167, and Japanese Unexamined Patent Publication No. 11-315231, adding a hydrolyzable silane compound (organic silicon compound) to ink in order to improve the water-resistivity of an image formed on a recording medium has been conventionally proposed. When a drop of the ink is adhered onto a recording medium, and a solvent contained in the ink drop, such as water, or the like, evaporates or permeates into the recording medium, the silane compound is condensation-polymerized, and this condensation-polymerized silane compound encloses a colorant. Thus, even when the image formed on the recording medium is exposed to water, the colorant does not exude into the water. That is, the water-resistivity of the image is improved.
However, in the case where such ink containing a hydrolyzable silane compound is used as it is, when an ink drop adhered onto a recording medium, the speed of evaporation of a solvent, such as water, from the ink drop and the speed of permeation of the solvent into the recording medium are relatively slow, and the condensation-polymerization of the silane compound is not quickly performed. As a result, enclosure of the colorant by the silane compound is insufficient. Especially when an image formed on the recording medium is exposed to water immediately after the formation thereof, a colorant which is not enclosed by the silane compound exudes into the water. In such a case, sufficient improvement of water-resistivity cannot be achieved.
The present invention was conceived in view of the above problems. An objective of the present invention is to surely improve the water-resistivity of an image formed with ink containing a water-soluble substance that is condensation-polymerized in the absence of water, such as a hydrolyzable silane compound. Especially, an objective of the present invention is to improve the water-resistivity of the image immediately after the formation thereof.